Someone

package huiswerk3;
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
/**
 *
 * @author Martijn
 */
public class Rekenmachine extends JFrame{
    private JPanel textpanel;
    private JTextField textvak;
    private JPanel knoppenpanel;
    private Knoppen knoppen;
    public Rekenmachine(){
        knoppen = new Knoppen();
        textpanel = new JPanel();
        knoppenpanel = new Knoppen();
        textvak = new JTextField(15);
        textvak.setEnabled(false);
        textvak.setText("0" + "aawaef"); // this works 
        textpanel.add(textvak);
        textpanel.add(knoppenpanel);
       setContentPane(textpanel);
    }
    public void setTextVak(){
        textvak.setText("0" + "aawaef"); // this doesnt work
        System.out.println("test"); // this works also
    }

}